The periglacial environment
159
them. The experimental studies indicate that the sorting increases with moisture and when
freezing is slow.
3.2. Chemical weathering
The presence of extensive accumulations of frost-shattered clasts on surfaces of variable
inclination is very common in periglacial environments, indicating that congelifraction is
the main weathering process. Although this process primarily generates large clasts, it may
also produce clay-size particles, as has been demonstrated with experiments in climatic
chambers (Guillien and Lautridou, 1970).
Because water remains in a solid state except during the melting period, the activity of
chemical weathering is generally limited (Hall et al., 2002). Besides, the kinetics of most
of the chemical reactions is slowed at low temperatures. However, rounded blocks of
diabase (basic igneous rock) and sandstones with exfoliation due to hydration and
oxidation processes have been locally recognized (Czeppe, 1964). In some cases chemical
weathering features are interpreted by some authors as indicators of past nonperiglacial
conditions. In Mediterranean mountain environments as in the Sierra Nevada of Spain,
the chemical weathering during the warm season on mica-schists is considered to be
important in the production of silts (A. G6mez Ortiz, personal communication). In coastal
environments, the presence of sodium-rich saline efflorescences resulting from the
evaporation of seawater transported by the wind is relatively common on the rocks. Melt
waters help the percolation of these saline waters into the rocks and the subsequent
precipitation of the dissolved ions may cause their fragmentation and granular
disintegration. This salt weathering process (haloclasty) is attributed to the genesis of
honeycomb and tafoni weathering, together with the wind action that helps to evacuate the
particles resulting from the disintegration of the rock (Washburn, 1969; Selby, 1972).
Nevertheless, the activity and intensity of chemical weathering in the periglacial zones
is not very well known due to scarce research carried out in this field. Surprising results
have been obtained in an 8-year long study conducted in an area with mica-schists and
amphibolites in the north of Sweden (Rapp, 1986). Here 48% of the weathering products
were evacuated in solution by the runoff. Other investigations indicate very low values for
this type of transport.
The solubility of carbonate rocks increases with the carbon dioxide content of the
water, which in turn reaches the maximum solubility in water at 0~ For this reason the
conditions of the periglacial environments favour the dissolution of carbonate rock.
The solution rates, however, are lower than in warm areas due to the scant biotic activity,
which is generally the main source of CO2, and the fact that the water may remain in a
solid state during the greater part of the year. The ice and the snow supply liquid water
by melting, and the karstification of the carbonates produces karren, doline fields and
cave systems, especially in alpine environments that may lead to the generation of
collapse sinkholes. In areas of continuous permafrost the melting water is not able to
penetrate into the ground and flows on the surface. Where the permafrost is discontinuous,
small caves may develop with a rare development of stalactites and stalagmites
(Sweeting, 1972).
159
them. The experimental studies indicate that the sorting increases with moisture and when
freezing is slow.
3.2. Chemical weathering
The presence of extensive accumulations of frost-shattered clasts on surfaces of variable
inclination is very common in periglacial environments, indicating that congelifraction is
the main weathering process. Although this process primarily generates large clasts, it may
also produce clay-size particles, as has been demonstrated with experiments in climatic
chambers (Guillien and Lautridou, 1970).
Because water remains in a solid state except during the melting period, the activity of
chemical weathering is generally limited (Hall et al., 2002). Besides, the kinetics of most
of the chemical reactions is slowed at low temperatures. However, rounded blocks of
diabase (basic igneous rock) and sandstones with exfoliation due to hydration and
oxidation processes have been locally recognized (Czeppe, 1964). In some cases chemical
weathering features are interpreted by some authors as indicators of past nonperiglacial
conditions. In Mediterranean mountain environments as in the Sierra Nevada of Spain,
the chemical weathering during the warm season on mica-schists is considered to be
important in the production of silts (A. G6mez Ortiz, personal communication). In coastal
environments, the presence of sodium-rich saline efflorescences resulting from the
evaporation of seawater transported by the wind is relatively common on the rocks. Melt
waters help the percolation of these saline waters into the rocks and the subsequent
precipitation of the dissolved ions may cause their fragmentation and granular
disintegration. This salt weathering process (haloclasty) is attributed to the genesis of
honeycomb and tafoni weathering, together with the wind action that helps to evacuate the
particles resulting from the disintegration of the rock (Washburn, 1969; Selby, 1972).
Nevertheless, the activity and intensity of chemical weathering in the periglacial zones
is not very well known due to scarce research carried out in this field. Surprising results
have been obtained in an 8-year long study conducted in an area with mica-schists and
amphibolites in the north of Sweden (Rapp, 1986). Here 48% of the weathering products
were evacuated in solution by the runoff. Other investigations indicate very low values for
this type of transport.
The solubility of carbonate rocks increases with the carbon dioxide content of the
water, which in turn reaches the maximum solubility in water at 0~ For this reason the
conditions of the periglacial environments favour the dissolution of carbonate rock.
The solution rates, however, are lower than in warm areas due to the scant biotic activity,
which is generally the main source of CO2, and the fact that the water may remain in a
solid state during the greater part of the year. The ice and the snow supply liquid water
by melting, and the karstification of the carbonates produces karren, doline fields and
cave systems, especially in alpine environments that may lead to the generation of
collapse sinkholes. In areas of continuous permafrost the melting water is not able to
penetrate into the ground and flows on the surface. Where the permafrost is discontinuous,
small caves may develop with a rare development of stalactites and stalagmites
(Sweeting, 1972).